In overseas countries, probabilistic risk assessment of nuclear power plant is based on probabilistic seismic hazard analysis (PSHA) applying the SSHAC (Senior Seismic Hazard Analysis Committee) Level 3. The Ikata SSHAC project is the first attempt in Japan to apply the SSHAC Level 3 seismic hazard analysis to a nuclear power plant, the Ikata Power Plant Unit 3. The characteristics of the Ikata site is that the Median Tectonic Line fault zone is located near the site and the hard rock site. Therefore, in order to take into account the epistemic uncertainty of the seismic motion, we introduced both ground motion prediction equation (GMPE) and fault rupture model simulation for evaluating ground motions, and analyzed the variations in ground motion near the seismic source. The results of this study are significant from the viewpoint of accurate and objective evaluation of uncertainty, and should be extended to subsequent PSHA.

The 2011 off the Pacific coast of Tohoku Earthquake and the 2016 Kumamoto Earthquake have increased the interest in the seismic hazard assessment. As new knowledge has been revealed with every new major earthquake, it is important to consider the range of the uncertainty of the parameters which are needed to the assessment and to understand the impact of each earthquake by probabilistic seismic hazard analysis, in advance. The Ikata SSHAC project is the first attempt in Japan to apply SSHAC Level 3 to the probabilistic seismic hazard analysis. In this paper, we report on the results of the modeling of various seismic sources, which include the Median Tectonic Line fault zone which is a long active fault and trench earthquakes occurring in the Nankai Trough. In addition, we discuss the effectiveness of guidelines for SSHAC Level 3, the range of the uncertainty and on the impact on the seismic hazard, because the gained knowledge and know-how of the scope would be useful for subsequent studies.

Fault displacement is an important hazard to be considered in the seismic design of structures, along with strong ground motion. In recent years, standards and safety guides for probabilistic fault displacement hazard analysis have been published, and social demand for probabilistic verification of fault displacement underneath critical facilities has increased. However, the practice of this method is very limited worldwide, and there are no reports of its full-scale implementation for a specific structure in Japan. In order to achieve wide application in the future, we should first practice probabilistic fault displacement hazard analysis based on a model with higher accuracy, and then understand the current issues properly. Here, we practice the probabilistic fault displacement hazard analysis using model of seismic source characteristics at the Ikata site based on guidelines for SSHAC Level 3, and report on the specific issues in its application at the practical level, referred to the knowledge obtained from the observation data of the 2016 Kumamoto earthquake.

To solve the problem of evaluating the area of SMGA data S_SMGA, which was a problem in an examination of the strong motion generation area (SMGA) data in the previous paper (Tohdo et al., 2022), the area S_SMGA was evaluated directly from SMGA data in a mean sense. In this evaluation, S_SMGA was scaled with M₀^2/3, M₀^1/2, and M₀¹ for seismic moment M₀, associated with the three-stage model in the "Recipe" by HERP. The empirical equation for the relationship of M₀-S_SMGA was established by applying the least-squares method to the SMGA data, and the stress drop calculated using this equation and the pre-set empirical equation for the short-period level was constant regardless of M₀ for each stage of the three-stage, and it was confirmed that they were consistent with the SMGA data.